1. Field of the Invention
This invention relates to a gas monitoring method and apparatus, and more particularly to a method and apparatus for the determination and continuous monitoring of the degree or percentage purity of a gas stream. In a particularly preferred embodiment, the invention relates to a method and apparatus for monitoring the hydrogen or oxygen content of a gas stream.
2. Prior Art
In the detection or measurement of constituents of a gaseous mixture it has been known heretofore to compare the thermal conductivity of one gas stream with that of a gas of a known composition or so-called "standard" or "reference" gas. In making such a comparison, it is customary to employ two electrically heated thermally sensitive elements, such as temperature responsive resistors connected to the arms of a Wheatstone bridge. One of the resistance elements is in contact with the gas under observation, and the other is positioned in a sealed container of the standard or reference gas. Such a method has not been altogether satisfactory. The equilibrium temperature attained by the thermally sensitive elements depends principally upon the heat loss rate of each element and the ability of gas surrounding each element to conduct heat; the temperature being lower when the gas has a high thermal conductivity and higher when the gas has a low conductivity. If the temperature sensitive element has a high temperature coefficient of electrical resistance, this resistance will have a value dependent upon the gas thermal conductivity.
Thus, if two substantially identical temperature sensitive resistance elements are exposed to gases having different thermal conductivities, the quantities of heat given off or the rate of heat loss of each of these elements to the respective gases will be different. This difference will result in one of the resistance elements being maintained at a higher temperature than the other. The difference in temperature will in turn result in a difference in the resistance of the two elements, thereby causing deflection of a galvanometer in the bridge circuit and necessitate an adjustment of the bridge to produce a state of balance. The magnitude of adjustment will, of course, be dependent upon the differences between the thermal conductivities of the two gases.
An example of a gas analysis apparatus in which the need for a reference gas has been eliminated is shown in U.S. Pat. No. 2,296,030. In accordance with the apparatus disclosed therein, both of the temperature sensitive devices are immersed in spaced relation in the gas under investigation. A perforated shield or baffle member is mounted between the two temperature sensitive devices to reduce radiation. Heat is applied to one of the temperature sensitive devices and under such condition that the temperature variation between the two devices is a function of the thermal conductivity of the gas. The ratio of the temperatures is measured by any suitable means such as a mercury thermometer or temperature sensitive resistance elements connected in a bridge circuit.
Thermal conductivity devices have not been altogether satisfactory. The reliability and accuracy of such devices are affected by numerous variables such as changes in flowrates of the gas stream, pressure, concentration of water vapor present, as well as other changes in the gas composition, all of which affect the thermal conductivity of the gas. Thus, the accuracy of such devices generally is poor, and the electrical circuitry required to attempt to compensate for all the variables is quite complicated.
In U.S. Pat. No. 3,567,394 there is depicted a system for determining impurity concentrations in a gaseous atmosphere. A known impure gaseous stream and an unknown impure gaseous stream are passed through parallel conversion zones to produce separate conversion product streams. The separate conversion product streams then are passed through detection devices, and the signal generated from such devices is quantitatively correlated with the impurity content of the unknown stream. The disadvantage of the disclosed system is that it requires a gaseous stream with a known impurity and in addition involves complex multiple flow paths.
U.S. Pat. No. 3,057,693 discloses yet another method and apparatus for monitoring mixed gas streams. In the apparatus disclosed therein, a container in which thermister bolometer flakes are mounted, is provided with inlet-outlet ports for receiving a small flow of a gas stream which is to be monitored. One or both of the flakes are covered with thin diaphragms which may be in the form of tiny cups with suitable electrical insulation. On one of the two diaphragms, or a single one if only one is used, there is placed a small amount of finely divided solid which reacts with the constituent in the gas stream which is to be monitored or which is a catalyst for the reaction of the constituent either with itself or with other constituents of the gas stream.
The reaction of the constituent of the gas to be detected produces heat or removes it and so changes the temperature of the thermistor flake with which it is in heat-conducting contact, whereas the other flake adjusts itself to the average temperature of the gas stream. If there is any of the constituent to be monitored present in the gas stream, one of the flakes becomes hotter than the other and a signal results which, with conventional electronic processing, can be used for an indicating, recording, alarm or control means. A disadvantage of such a system is that it requires flow regulation and appears to be an inherently slow response, high drift device.
In U.S. Pat. No. 2,916,358 there is disclosed an apparatus for detecting carbon monoxide. The apparatus comprises a conduit through which the atmosphere to be tested is passed. Also, located within the conduit member is a thermistor juxtaposed with an oxidizer and another thermistor juxtaposed with an inert material. The inert material and the oxidizer are sequentially contacted with the atmosphere to be tested. The thermistors are connected in an electrical circuit adapted to produce a signal indicative of the difference in temperature between the thermistors, and hence the quantity of carbon monoxide in the atmosphere. An essential part of the disclosed apparatus is a coil circumscribing the conduit member and in heat exchange relationship therewith, whereby the atmosphere to be tested passes through the coil and absorbs heat from the oxidizer zone prior to its entry into the conduit member. With such an arrangement the temperature of both thermistors will increase with increasing reactant concentrations. This, in turn, results in a slow response such that the apparatus would not appear capable of accurate quantitative measurement of fluctuating reactant concentrations.
In many chemical reactions or physical separations there is a production of continuous streams of mixed gaseous components, and it frequently is desirable or even necessary to determine the amount of one component which may be present in the gas stream in relatively small amounts. There still is need for an apparatus for such applications. Further, such apparatus should be capable of providing an accurate continuous response. There is a particular need for method and apparatus to continuously and reliably monitor the presence of oxygen or hydrogen, whichever is the lesser stoichiometric constituent and where both are present, in a gas stream. An application for such an apparatus exists, for example, in the field of water-cooled nuclear reactors where metal-water reactions and decomposition of the cooling water can occur to form free hydrogen and oxygen. The hydrogen and oxygen must be recombined, either for collection and return to the reactor containment or for disposal, to prevent the hydrogen concentrations reaching a combustible limit, which is about 4% hydrogen in air. Therefore, it is essential to have a reliable hydrogen/oxygen analyzer available to accurately indicate the amount of hydrogen present in the containment atmosphere.